1. Field of the Application
The present application relates to a multi-reflection structure and a photo-electric device. More particularly, the present application relates to a multi-reflection structure that reduces surface reflection and a photo-electric device having the same.
2. Description of Related Art
Recently, photo-electric industries (display industry, solid-state illumination device industry, solar cell industry, and so on) grow rapidly and continuously change living habits of human beings. However, during research and development of the aforesaid photo-electric industries, surface reflection issue resulted from difference of refractive indices is inevitable. In displays, overall brightness thereof is reduced by surface reflection. In solid-state illumination devices (e.g., light-emitting diodes or organic electro-luminescent devices), illumination performance thereof is reduced by surface reflection. According to some researches, in organic electro-luminescent devices, about 70% to 80% optical loss is resulted from surface reflection.
Similarly, in the solar cells, photoelectric conversion efficiency thereof is reduced by surface reflection also. Specifically, solar cells are kinds of photo-electric devices that convert light into electric power. The photoelectric conversion efficiency of solar cells is related to photo current and voltage generated therefrom. In order to increase photo current of solar cells, light absorption of solar cells is required to be increased. Since the conventional mono-crystalline silicon solar cells have sufficient thickness, light absorption of mono-crystalline silicon solar cells is not a problem. Accordingly, it is imperative to reduce optical loss resulted from surface reflection of mono-crystalline silicon solar cells.
According to the Fresnel's Law, when light propagates through an interface of two mediums having different refractive indices, reflectivity of the propagated light is proportional to difference of refractive indices. Specifically, the smaller the difference of the refractive indices of the two mediums is, the lower the reflectivity of the propagated light can be obtained. On the contrary, the greater the difference of the refractive indices of the two mediums is, the higher the reflectivity of the propagated light can be obtained. Take silicon substrates that are often used in semiconductor devices as an example, refractive index thereof is about 3 to 4. When light propagates the interface of air and the silicon substrate having a flat surface, reflectivity of the propagated light is considerably high (e.g., reflectivity is about 36%).
In the conventional solar cells, a hydrogen containing amorphous silicon nitride serving as an anti-reflection coating is suggested to be formed on the solar cells to reduce surface reflection issue and enhance photoelectric conversion efficiency of solar cells. However, the anti-reflection coating cannot significantly enhance photoelectric conversion efficiency of solar cells. According, some prior arts (e.g., U.S. Pat. No. 5,081,049, U.S. Pat. No. 5,080,725, US 2009/071536, TWM 354858, and U.S. Pat. No. 7,368,655) have proposed. In the aforesaid prior arts, Optical micro-structures are suggested to be formed on a light-incident surface of solar cells, such that light incident from the light-incident surface of solar cells can be reflected twice and optical loss resulted from surface reflection can be reduced. However, in the aforesaid prior arts (e.g., U.S. Pat. No. 5,081,049, U.S. Pat. No. 5,080,725, US 2009/071536, TWM 354858, U.S. Pat. No. 7,368,655), since almost light is reflected by the optical micro-structures twice, optical loss resulted from surface reflection cannot be reduced significantly.
As such, it is imperative to further reduce optical loss resulted from surface reflection.